What is positron emission tomography (PET) scan?
Positron emission tomography (PET) is a type of nuclear medicine procedure that measures the metabolic activity of cells in body tissues. PET is actually a combination of nuclear medicine and biochemical analysis. Primarily used in patients with brain or heart disease and cancer, PET scans help visualize the biochemical changes that occur in the body, such as metabolism (the process by which cells turn food into energy after food is digested) and how it is absorbed. in the blood) of the heart muscle
PET differs from other nuclear medicine scans in that PET detects the metabolism within the body's tissues, whereas other types of nuclear medicine scans detect the amount of a radioactive substance that has accumulated in body tissue at a particular location to examine the function of the fabric.
As PET is a type of nuclear medicine procedure, this means that a small amount of a radioactive substance, called a radiopharmaceutical (radionuclide or radioactive tracer), is used during the procedure to help examine the tissue being studied. Specifically, PET studies evaluate the metabolism of a given organ or tissue, thus evaluating information about the physiology (functionality) and anatomy (structure) of the organ or tissue, as well as its biochemical properties. Thus, PET can detect biochemical changes in an organ or tissue that can identify the onset of a disease process before disease-related anatomical changes can be seen with other imaging processes such as computed tomography (CT) or nuclear magnetic resonance (NMR) ).
PET is most commonly used by oncologists (doctors who specialize in the treatment of cancer), neurologists and neurosurgeons (doctors who specialize in the treatment and surgery of the brain and nervous system), and cardiologists (doctors who specialize in the treatment of the heart). However, as advances in PET technologies continue, this procedure is starting to be used more widely in other areas.
PET can also be used in conjunction with other diagnostic tests such as computed tomography (CT) or magnetic resonance imaging (MRI) to provide more definitive information about malignant (cancerous) tumors and other lesions. The latest technology combines PET and CT into a single scanner known as PET/CT. PET/CT shows particular promise in the diagnosis and treatment of lung cancer, evaluating epilepsy, Alzheimer's disease and coronary artery disease.
Originally, PET procedures were performed in dedicated PET centers because, in addition to the PET scanner, the equipment to manufacture the radiopharmaceuticals, including a cyclotron and a radiochemistry laboratory, had to be available. Radiopharmaceuticals are now produced in many areas and shipped to PET centers, so only the scanner is needed to perform a PET scan.
Further increasing the availability of PET imaging is a technology called gamma camera systems (devices used to scan patients who have been injected with small amounts of radionuclides and currently in use with other nuclear medicine procedures). These systems have been adapted for use in PET scanning procedures. The gamma camera system can complete a scan more quickly and at a lower cost than a traditional PET scan.
How does PET work?
PET works by using a scanning device (a machine with a large hole in the center) to detect photons (subatomic particles) emitted by a radionuclide in the organ or tissue being examined.
The radionuclides used in positron emission tomography (PET) scans are made by attaching a radioactive atom to chemicals that a particular organ or tissue naturally uses during its metabolic process. For example, in PET scans of the brain, a radioactive atom is applied to glucose (blood sugar) to create a radionuclide called fluorodeoxyglucose (FDG), because the brain uses glucose for its metabolism. FDG is widely used in PET scanning.
Other substances may be used for the PET scan, depending on the purpose of the scan. If blood flow and perfusion of an organ or tissue is of interest, the radionuclide can be a type of radioactive oxygen, carbon, nitrogen or gallium.
The radionuclide is given into a vein through an intravenous (IV) line. The PET scanner is then slowly moved over the body part being scanned. Positrons are emitted by the decay of the radionuclide. Gamma rays called annihilation photons are created when positrons collide with electrons near the decay event. The scanner then detects the annihilation photons, which arrive at the detectors coincidentally 180 degrees apart from each other. A computer analyzes these gamma rays and uses the information to create an image map of the organ or tissue being studied. The amount of radionuclide collected in the tissue affects the brightness of the tissue in the image and indicates the level of organ or tissue function.
Why is PET performed?
In general, PET scans can be used to evaluate organs and/or tissues for the presence of disease or other conditions. PET can also be used to assess the function of organs such as the heart or brain. The most common use of PET is in detecting cancer and evaluating cancer treatment.
More specific reasons for PET scans include, but are not limited to:
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To diagnose dementias (conditions involving impaired mental function) such as Alzheimer's disease, as well as other neurological conditions such as:
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Parkinson's disease. A progressive disease of the nervous system in which a slight tremor, muscle weakness and a peculiar type of gait is observed.
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Huntington's Disease. An inherited disease of the nervous system that causes increased dementia, awkward involuntary movements, and abnormal postures.
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Epilepsy. A brain disorder that involves recurrent seizures.
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cerebrovascular accident (stroke)
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To locate the specific surgical site prior to brain surgical procedures
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To evaluate the brain after trauma for hematoma (blood clot), bleeding, and/or perfusion (flow of blood and oxygen) of brain tissue
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To detect cancer spread to other parts of the body from the original cancer site
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Evaluate the effectiveness of cancer treatment.
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Evaluate perfusion (blood flow) to the myocardium (heart muscle) to help determine the usefulness of a therapeutic procedure to improve blood flow to the myocardium
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To better identify lung lesions or masses detected on chest X-ray and/or chest CT
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Assist in the management and treatment of lung cancer by staging the lesions and monitoring the progress of the lesions after treatment.
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To detect tumor recurrence earlier than with other diagnostic modalities
How is PET performed?
PET scans can be performed on an outpatient basis. Some hospitalized patients may also have a PET scan for certain conditions.
While each center may have specific protocols, a PET procedure generally follows this process:
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The patient will be asked to remove any clothing, jewelry or other objects that might interfere with the exam.
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If asked to remove clothing, the patient will be given an apron to wear.
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The patient will be asked to empty their bladder before starting the procedure.
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One or 2 IV lines will be started in the hand or arm for radionuclide injection.
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Certain types of abdominal or pelvic scans may require insertion of a urinary catheter into the bladder to drain urine during the procedure.
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In some cases, an initial scan may be performed before radionuclide injection, depending on the type of study being performed. The patient will be positioned on a padded table inside the scanner.
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The radionuclide will be injected into the IV. The radionuclide will be able to concentrate in the organ or tissue for approximately 30 to 60 minutes. The patient will remain in the unit during this period. The patient will not be dangerous to others, as the radionuclide emits less radiation than a standard x-ray.
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After the radionuclide has been absorbed for the appropriate amount of time, scanning will begin. The scanner will slowly move over the body part being studied.
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When the scan is complete, the IV line will be removed. If a urinary catheter was inserted, it will be removed.